Make a map in the form of a physical, three-dimensional sculpture.

You can map any subject, in any material you like. Clay, wood, papier mache, construction toys, sand, cake icing, whatever. Just as long as it's something physical (not computer graphics) and 3D (so not simply hand-drawn artwork).

It's an out-there challenge, requiring you to most likely throw away all your normal mapping techniques and pursue something novel.